A vertical cavity surface emitting laser (VCSEL) is a type of laser diodes and emits light in a direction perpendicular to a substrate. VCSELs are characterized by their low manufacturing costs and are often used in optical interconnects. Applying the wavelength multiplexing technique using a plurality of VCSELs that emit light of different wavelengths to a Quad Small Form-factor Pluggable (QSFP) module makes it possible to increase the transmission band of an existing optical fiber.
QSFP modules are used, for example, for short-distance communications between servers or between switches in a data center. The application of the wavelength multiplexing technique enables wide-band optical signals to be transmitted by means of a single optical fiber. Hence, it becomes possible to implement wide-band transmission by utilizing an existing 2-core optical cable (duplex fiber) without additionally laying parallel optical fibers.
A general QSFP module allows 4-channel optical signals to be transmitted and received using 8 optical fibers included in an 8-core optical cable. If the wavelength multiplexing technique is applied to such a QSFP module to multiplex a 4-channel optical signal in one optical fiber, it becomes possible to transmit and receive 4-channel optical signals using a 2-core optical cable.
Regarding the wavelength multiplexing optical transmission, various optical control modules and optical multiplexers and demultiplexers are known.
Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication Nos. 2005-17811 and 2005-274700, International Publication Pamphlet No. WO 2006/134675, and the like.
In the case where the wavelength multiplexing technique using VCSELs is applied to a QSFP module, light emitted in a direction perpendicular to the substrate is to be multiplexed. For this reason, it is difficult to thin the QSFP module.
Such a problem is not limited to the QSFP modules using VCSELs but occurs also in other optical modules using other surface emitting elements. Such a problem is not limited to the optical modules using surface emitting elements but occurs also in optical modules using light receiving elements.
In view of these, it is desirable to thin an optical module using surface emitting elements or light receiving elements.